Switches & Blinky Light Front Panel for PC?

Yup... best to use the JTAG/debug interface. There's silicon in the current PC CPUs dedicated to off chip debugging, and best of all, requires only a handful of signals connected (a dozen maybe) instead of tying off of each address/data/control signal from the processor (which you may or may not have access to). Best of all, you can leave a processor halted for days if you wish. ________________________________ From: Guy Sotomayor <ggs at shiresoft.com> To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org> Sent: Tue, November 24, 2009 11:07:18 AM Subject: Re: Switches & Blinky Light Front Panel for PC? On Nov 24, 2009, at 8:38 AM, Sridhar Ayengar wrote: Because the processor is still driving the bus. The /WAIT pin isn't the right way to do it either. There's a DMA request pin (forgot the name at the moment) that will cause the processor to relinquish control of the bus. That will allow you to "halt" it and read/write memory from the front panel (hey look a DMA front panel!). This is what I did (mumble) years ago. I'd have to go back and dig out the designs (I still have them some place). As I recall, to start at a particular address, it jammed a jump instruction (and address) on to the bus. Single stepping *was* done with the /WAIT pin. Doing this with anything much later than 486 is nigh impossible (and even that is problematic) without *a lot* of seriously designed HW. The buses are fast and have special signaling requirements (GTL, GTL+, etc). Don't even think about modern (last 5 or so years) x86 CPUs. In some of our labs we have bus analyzers that plug into the bus (can't do it with the new Nehalem CPUs). Those are $500K+ each (and are only good for that type of CPU) and each time I look at them, I'm amazed they work as well as they do. TTFN - Guy